A stent is an expandable prosthesis that can be delivered into a body vessel or passageways such as blood vessels, respiratory ducts, gastrointestinal ducts, urinary vessels, and the like. Stents have been employed to treat a host of diseases and disorders, including abdominal aortic aneurysms, coronary artery disease, and blockage of the bile duct. These devices are typically deployed in a compressed state using a catheter, of which there are many different types. In the case of arterial disease, a catheter can be guided through a patient's arterial system, until the catheter's distal end reaches a desired location within the patient, typically a constriction or lesion in an artery. Once the catheter is correctly positioned inside the artery, the stent can be released. During the deployment process the stent is converted from a compressed state to an expanded state that serves to provide support to and/or keep open the artery.
Stents can generally be divided into two types with regard to the manner in which they are converted from the compressed state to the expanded state. These groups are self-expanding stents and balloon expandable stents. Self-expanding stents, as the name suggests, will automatically expand from the compressed state to the expanded state when they are released from the catheter. Balloon expandable stents, on the other hand, are mounted on the exterior of a balloon that is located toward the distal end of the catheter. Conversion from the compressed state to the expanded state is achieved by inflating the balloon, which concomitantly expands the balloon expandable stent.
One drawback commonly associated with self-expanding stents is that they must be compressed from the expanded state to a compressed state so that they can be loaded into the catheter. Compressing these stents typically strains the stent and also creates radial force. The amounts of strain and radial force created will depend on the specific design of the stent, the materials from which the stent is constructed, and the extent to which the stent is compressed. In many cases, the amount of strain and the amount of radial force increase as the stent is compressed to smaller diameters. Eventually, the strain may become so severe that the stent will undergo permanent deformation or failure. As a result, this strain may limit the degree to which the stent can be compressed. Since the amount of radial force increases as the stent is compressed to smaller diameters, it becomes progressively more difficult to compress these stents to smaller diameters. Thus, it may be difficult to compress these stents to the desired diameter, especially when a smaller diameter is desired. Furthermore, the increased radial force makes it much more difficult to release the compressed stent from the catheter, since the amount of radial force present is directly proportional to the amount of friction that will occur between the compressed stent and the inside of the catheter.
Another problem with many of the current designs is that they have a short fatigue life. In terms of a stent, the fatigue life is the number of cycles of compression/expansion that the stent can undergo before it fails or permanently deforms. For example, arterial stents undergo cycling due to normal blood flow through a patient's blood vessels. With every heart beat, the heart creates a surge of blood that pulses through the blood vessels, causing them to expand. Once this surge of blood passes, the blood vessel contracts. Thus, the stent is continuously compressed and expanded. In many current stent designs, the stresses created by this cycling are focused at specific regions within the stent and consequently these regions are the first to permanently deform.
Ideally, a stent would be capable of more evenly distributing the strain associated with cycling over a greater area of the stent. This in turn should lower the peak magnitude of strain, resulting in a stent with a greater fatigue life. In addition, a stent capable of more evenly distributing the strain associated with cycling over a greater area of the stent should be capable of being compressed to fit within a low-profile catheter. Furthermore, an ideal stent would have a wide range of use, in that it would be capable of being used for a range of diameters.